The present invention relates to a master cylinder of a braking circuit, particularly for a motor vehicle.
Motor vehicles comprise a brake control generally comprising a brake pedal acting on a master cylinder, to transmit, via a hydraulic circuit, a fluid pressure to receivers that brake the various wheels.
The master cylinder comprises a piston sliding axially in a body comprising a lip seal which presses against the piston, in order to separate a rear volume containing a reserve of liquid at atmospheric pressure from a front volume comprising a chamber pressurized by the displacement of this piston.
A master cylinder of this type, described in document U.S. Pat. No. 6,272,858 B1, is equipped with a circular seal with two lips. The internal lip of the seal surrounds the piston providing dynamic sealing and, when the piston is at rest in a rear position, enters a piston groove which has a shallow depth and gently inclined side walls.
In this piston rest position, a radial drilling formed in the piston opens into the groove behind the internal lip of the seal, to place the pressure chamber in communication with the reserve of liquid and, if necessary, readjust the volume of the hydraulic circuit.
The lip seal is wedged axially in an annular housing of the body around the piston and comprises an external lip which presses against an external cylindrical wall of this housing, to provide static sealing between the pressure chamber and the rear volume.
The seal additionally comprises a planar rear face which presses against a planar transverse rear face of the annular housing, to wedge the seal axially in the housing.
Motor vehicles may comprise driving aid systems which brake some of the wheels of the vehicle, for example to stabilize the course followed by the vehicle, in the case of ESP systems.
In such cases, the system may quickly call for substantial amounts of liquid in the brake circuit, which liquid is drawn through the master cylinder to be supplied to the wheel brakes, from the reserve of brake fluid. The fluid has then quickly to pass via the lips of the annular seal, which lift because of the depression pulled downstream.
In an known arrangement, the rear face of the annular seal comprises radial grooves which extend over the entire height of the seal, so as to allow liquid to pass radially outwards, between the annular seal and the planar rear face of the housing of the body.
When there is a demand for fluid in the brake circuit, the liquid follows a path along the radial grooves to reach the external lip, and feeds into the braking circuit via this path.
In the case of an annular seal that has been produced by molding, one known method of manufacturing the mold is to use EDM machining to machine that surface of the mold that corresponds to the rear face of the annular seal, comprising the bearing surfaces and the grooves, using a one-piece electrode.
A chief disadvantage with this method, given the spread on manufacturing dimensions, is that it is possible that the electrode may not be positioned precisely relative to those parts of the mold that have been produced beforehand by turning, and this may give rise to leaks in the annular seal at the lines where the surfaces created by turning meet those created by EDM, when such lines are on a dynamic sealing surface.